Multiple-layer insulating glass windows and similar units comprise at least two parallel panes of glass spaced apart to create an insulating air space between the panes. Usually the parallel panes are sealed around their periphery with an aluminum spacer tube to form an integrated structure having no communication between an internal cavity between the panes and the outside atmosphere. In the fabrication procedure, at the time the panes are sealed to the spacer tube, ambient air containing water vapor is entrapped in the internal cavity and, depending upon the particular sealant employed, organic species emanating from the sealant, such as solvent vapors and low molecular weight polymers, can also be present. Condensate of these vapors tends to collect on the inner surfaces of the sealed glass panes and to create a visual and/or aesthetic impairment which cannot be remedied except by extreme measures. For example, U.S. Pat. No. 4,144,196 to Schoofs discloses an improvement in sealed insulating glass windows by disposing an adsorbent about all or part of the interior periphery of the glass. The improvement lies in employing a molecular sieve zeolite, such as zeolite A, that permits adsorption of water vapor and prevents adsorption of nitrogen and oxygen as the adsorbent. U.S. Pat. No. 4,151,690 discloses that the adsorbent preferred for water adsorption is zeolite X.
U.S. Pat. No. 3,868,299 to Ulisch discloses the use of an adsorbent designed for use in multiple layer insulating glass windows comprising a narrow-pore zeolite in combination with a wide-pore adsorbent and, optionally, a clay binder. Ulisch discloses that narrow-pore zeolites include sodium zeolite A, and wide-pore adsorbents include faujasite (zeolite X and Y), active carbon, silica gel, aluminum oxide and mixtures thereof. Kaolin, attapulgite, bentonite, waterglass, gellable silica sols and mixtures thereof are disclosed as suitable binders. Ulisch discloses that the adsorbent materials are prepared by methods known in the art as oil dropping, wherein a suspension of the adsorbent materials is stirred with an aqueous stable silica sol to form a free-flowing suspension which is subsequently introduced to a liquid immiscible with water to form droplets of the required size. The bead granulates formed are removed from the liquid as completely homogeneous bead granulates. The bead granulates are screened, dried, and dehydrated in hot air at about 350.degree. C. Bead granulates produced in this manner represent the most common beads for use in insulating glass windows and, typically, have bulk densities ranging from about 0.640 gm/cc (40 lbs/cf) to about 0.960 gm/cc (60 lbs/cf) and range in size from about 10 to about 40 mesh.
Others such as U.S. Pat. No. 5,132,260 to Plee disclose the preparation of adsorbents for use in multi-pane windows by making a paste from the zeolite powder, a silica gel and a sodium aluminate solution and mechanically shaping the adsorbent at room temperature, followed by heat treating at 50.degree.-100.degree. C. and calcination at 450.degree.-600.degree. C. The resulting zeolite agglomerate has superior water adsorption and mechanical strength.
U.S. Pat. No. 4,476,169 relates to an adsorbent material for multilayer glazing wherein an adsorbent in the spacers consists of a granular zeolite having a core of synthetic zeolite with a clay binder having more zeolite than the particle as a whole, and a shell of synthetic zeolite with a clay binder, wherein the shell has more clay than in the particle as a whole, and particles of activated carbon coated with synthetic latex.
As in essentially all adsorption procedures, it is the current practice of manufacturers of adsorbent agglomerates for use in dual pane windows to maximize the adsorbent capacity of each agglomerate. Thus, the filling of substantially all portions of spacer tubing with currently available adsorbents for use in windows results most often in the significant overuse of expensive adsorbent.
A number of seemingly simple and obvious solutions to the excessive adsorbent problem may suggest themselves. For example, decreasing the size of the spacer tube would decrease the quantity of adsorbent necessary to fill it. The basic functions of the spacer tube, however, are to control the spacing of the panes and provide the structure and the surfaces for sealing the panes to form an integrated unit. A decrease in spacer size would have an adverse effect upon at least one of these functions. It is, however, necessary that any portion of the spacer tube which contains adsorbent particles be substantially completely filled in order that the particles do not move and abrade each other during handling, transportation, and installation with the consequent production of dust in the internal cavity of the window.
Another approach which would decrease the amount of adsorbent and the cost is to employ an inexpensive diluent. The apparent logic of incorporating an inexpensive inert diluent into the adsorbent agglomerate ceases to be compelling when it is realized that there are no known diluents which are both inexpensive and have no substantial adverse effect upon the manufacture of the adsorbent particle. The function of the adsorbent agglomerate as an adsorbent with diluents such as wood, flour, or walnut shells, assuming they are sufficiently inexpensive, cannot tolerate the calcination procedures necessary for activation of the zeolite constituent. Inorganic diluents such as clays, aluminas and silicas have been proposed and actually used in prior art window desiccants. These materials are not at all inexpensive and, in fact, may even exceed the costs of certain zeolites. It is, moreover, of great importance that the diluent be capable of being incorporated into a desiccant particle in a manner which does not adversely affect the adsorbent properties of the zeolite constituent, i.e., binder-blinding, pore-clogging, and the like, and not create dusting problems or contribute to a lack of strength of the overall desiccant particle.
Methods were sought to expand the agglomerate to fill the entire spacer tube without adding additional costly zeolite adsorbent. Spray drying of the adsorbent agglomerate appeared to have the potential to decrease the bulk density of the adsorbent, but most of the art of spray drying of zeolites or clay was directed to the production of attrition resistant, concentrated particles. For example, in general, spray-drying involves the atomization of a suspension of material with drying air to form droplets. As soon as the droplets come into contact with the drying air, evaporation takes place from the outside surface of the droplets. According to K. Masters in Spray Drying Handbook, 5th Edition, John Wiley & Sons, Inc., 1991, pages 37-39 and 333-336 and 346-348, evaporation takes place in two stages. It the first stage, evaporation continues at the surface, while diffusion of moisture from within the droplet replenishes the moisture lost from the surface. This constant rate stage continues until the moisture content becomes too low to maintain the saturated condition at the surface. At this point, a dried shell forms at the droplet surface. Evaporation now continues at the rate of moisture diffusion through the dried surface shell. The thickness of the dried shell increases with time, causing the rate of evaporation to decrease resulting in the falling rate period of drying.
During evaporation, the spray distribution of the material within a spray droplet undergoes change. Different materials exhibit different evaporation characteristics which result in different types of particles. Some particles tend to expand, others collapse, fracture or disintegrate, leading to porous, irregularly shaped particles. The tendency for the formation of hollow particles by spray drying is described in Masters, ibid., pages 347-348, as a function of four mechanisms relating to (1) the expansion of the droplet with increasing temperature, (2) rapid evaporation rates at the interior of the droplet, (3) capillary action induced flow of liquid from the center of the droplet, and (4) entrainment of air.
U.S. Pat. No. 4,946,814 to Shi et al. discloses a process for significantly improving the physical and catalytic properties of the faujasite containing fluid cracking catalysts employing a sol binder by incorporating acid stable surfactants into the catalyst component slurry prior to spray drying to eliminate the "blow hole" and fragile shell character of the particles to produce stronger, more dense particles. Shi et al. is representative of the spray drying art which taught the empty particles with blow hole are to be avoided.